As is known, premix burners with total air/gas premixing are today widely used for producing thermal energy in gas boilers.
The use of these burners is rapidly spreading replacing traditional atmospheric burners in so far as, as compared with the latter, they enable:    [A] lower emissions of pollutant substances (nitrogen and carbon oxides);    [B] high heat-exchange efficiency at all thermal-power regimes and in particular at the minimum thermal power; and    [C] high modulation range between the maximum and the minimum thermal power of the burner.
Air/gas premix burners are today prevalently obtained using the following essential components:                a fan for delivery of the air/gas mixture to a combustion head;        a gas valve actuated pneumatically equipped with a flow regulator;        an air/gas mixing system constituted by a Venturi tube or by a diaphragm performing a similar function (see hereinafter); and        a combustion head provided with the device for ignition of combustion of the air/gas mixture.        
In these systems, the active device (also referred to as “driver”) is represented by the fan, which, being supplied electrically in an appropriate way, delivers the combustion air to the burner in an amount directly proportional to the thermal power that it is intended to supply to the burner and hence to the thermal power of the head of the burner.
The passive device (also referred to as “follower”) is represented by the gas valve, which is able to supply gas in an amount directly proportional to the amount of air blown into the system thanks to the regulation system illustrated hereinafter.
The gas valves are characterized in that, irrespective of the value of the pressure of the incoming gas (obviously, within the limits of work allowed by the valve itself and corresponding to the pressures of distribution of the mains-supply gas), they supply gas at output at a pressure equal to the pressure exerted on their “regulator”.
Explained in greater detail hereinafter are the aforesaid general concepts with reference to the attached figures, where:
FIG. 1 illustrates a first embodiment of a traditional premix burner; and
FIG. 2 shows a second embodiment of a premix burner of a known type.
In a burner 10 illustrated in FIG. 1 an air/gas mixer of a Venturi-tube type 11 is set downstream of a fan 12 with respect to an air flow (AF). The mixer 11 comprises a device for localized pressure loss 11A, in this case constituted by a Venturi tube.
Connected upstream of the Venturi-tube air/gas mixer 11 is a conduit 13 that sends a pressure signal P1 to a gas valve 14. In addition, entering the gas valve 14 is a flow of gas (FG) at the mains-supply pressure Po.
The amount of gas released by the gas valve 14 to the mixer 11 is correlated to the pressure difference existing between a pressure P2 at output from the gas valve 14 (pressure P2 equal to the value of the pressure P1) and a pressure P3 existing in the narrowest point (localized-pressure-loss device 11A) of the Venturi-tube air/gas mixer 11.
A flow regulator 15 set on a tube 16 for connection between the gas valve 14 and the Venturi-tube air/gas mixer 11 enables regulation of the amount of gas supplied so as to have an optimal air/gas ratio for combustion of the mixture in a combustion head (TC).
The system, once calibrated through adjustment of the flow regulator 15, enables a constant air/gas ratio to be obtained throughout the operating range of the burner 10.
It is evident, in fact, that, for any value of air flowrate generated by the fan 12, the pressure difference (P1−P3), generated by the air flowrate, and measured between the inlet and the narrowest section of the Venturi-tube air/gas mixer 11, will be the same as the one that will generate the rate of gas coming out of the gas valve 14, given that the Venturi-tube air/gas mixer 11 is a rigid and undeformable mechanical member.
The gas/air mixture is sent according to a flow (MF) towards the combustion head (TC). The burner 10 is completed by a device 17 for ignition of the flame and detection of the presence thereof, and by an electronic control unit (CNT), which controls operation of the fan 12, of the gas valve 14, and of the device 17 itself.
In a second embodiment known in the prior art and illustrated in FIG. 2, the Venturi-tube air/gas mixer 11 is located upstream of the fan 12.
It should be said incidentally that, in the second embodiment of FIG. 2, the same numbering of FIG. 1 has been used for designating elements that are identical or similar to the ones appearing in FIG. 1.
In this second embodiment the type of pressure signal P1* coincides with the atmospheric pressure Pa that acts simultaneously on the regulator 15 of the gas valve and in the inlet mouth of the Venturi-tube air/gas mixer 11.
The amount of gas released by the gas valve 14 is correlated to the pressure difference existing between the output pressure P2* (equal, in this case, to the atmospheric pressure Pa and to the pressure P1*) and the pressure P3* existing in the narrowest point of the Venturi-tube air/gas mixer 11.
Also in this case, the flow regulator 15 set on the conduit 16 for connection between the gas valve 14 and the Venturi-tube air/gas mixer 11 enables regulation of the amount of gas supplied so as to have an optimal air/gas ratio for combustion.
The system, once calibrated by means of the regulator 15, enables a constant air/gas ratio to be obtained throughout the operating range of the burner 10.
It is evident, in fact, that for any value of air flowrate generated by the fan 12 the pressure difference (Pa−P3*) (with Pa equal to the ambient pressure) generated by the air flow (AF) and measured between the inlet and the narrowest section of the Venturi-tube air/gas mixer 11 will be the same that generates the flowrate of gas coming out of the gas valve 14.
In actual fact, in order to improve combustion, the air/gas ratio is purposely not kept rigorously constant throughout the modulation range, but is varied by a few tenths of percentage point.
However, given that this variation is very small, it is altogether of no effect for the purposes of the present treatment.
A possible variant (not illustrated) with respect to both of the systems illustrated in FIGS. 1 and 2 is represented by the use of diaphragms as an alternative to the use of an air/gas mixer of a Venturi-tube type.
However, premix burners of the types described with reference to FIGS. 1 and 2 present the following disadvantages:                a modulation range that varies from 100% to 20% (ratio 1:5) of the nominal thermal power; and        high losses of head at the maximum thermal power.        
Consequently, the need has been felt to:                increase the modulation range so as to reach minimum values of 10% (ratio 1:10) and even lower; and        reduce the losses of head of current mixing systems.        
The first requirement arises from the fact that the premises to be heated present ever lower heat dispersions, whereas users have increasingly higher needs of comfort for production of hot water for sanitary purposes.
In addition, as has been said, there is an increasingly widespread use of boilers of a combined type (also referred to as “boilers of a combi type”), i.e., ones that are able to supply heat to the water of the heating system and, when required, to the hot water for sanitary uses.
This type of boiler must have, however, the capacity to supply continuously (i.e., without any turning-off of the burner) energy to a markedly differentiated extent, i.e., a very high extent for the production of water for sanitary purposes and a very limited extent for the production of heat for the heating system.
It is known, in fact, that the operation of a burner of an intermittent type is a source of dispersions of energy for managing transient phases of startup and turning-off (preventilation and/or postventilation for safety requirements) in addition to the emission of pollutants in the ignition step.
The modulation range is currently limited by some physical and technological limits of the systems, which can be summarized as follows:                the fans currently in use are able to function properly in a range comprised between 1000 and 6000 r.p.m.; above 6000 r.p.m. the efficiency of the fans drops drastically, whilst the problems of noise generated by the moving parts (impellers, bearings, air flow, etc.) increase considerably; furthermore, below 1000 r.p.m. the problems of stability of the velocity of rotation of the fan increase considerably, with consequent problems of combustion; in addition        the gas valves are currently able to function properly with values of pressure at input to the regulator of higher than 30÷40 Pascal.        
Below these values the problems of repeatability of the value of pressure at output from the gas valve increase considerably, with consequent marked variations in the air/gas ratio and hence with problems of flame lifting from the combustion head or of low level of combustion hygiene.
If we keep the minimum velocity of the fan referred to above constant, the Venturi tubes (or the diaphragms) are able to supply differences of pressure higher than the minimum ones required for the gas valves only on the condition of having a very small minimum section of passage. Consequently, even by pushing the fans to the maximum speeds allowed the maximum air flowrates that can be obtained (and hence, in the ultimate analysis, the maximum achievable thermal powers) are limited to not more than 5÷6 times the values of thermal power obtained at the minimum speed.
The second requirement of the users derives from the fact that it is possible to use in the production of the burner fans with lower performance and hence less costly given the same achievable modulation ratio.
In particular, the present invention finds advantageous, though non-exclusive, application in combination with a combined boiler for simultaneous or differed production of water for heating premises and of hot water for sanitary purposes.